Recently, from a viewpoint of preservation of a global environment, attempts have been made to reduce exhaust gas such as CO2. In automobile industries, there has been taken a countermeasure which lowers a quantity of exhaust gas by improving fuel consumption by making a vehicle body light-weighted.
As one of techniques for making the vehicle body light-weighted, there has been named a technique which decreases a sheet thickness by strengthening a steel sheet used for manufacturing an automobile. Further, a steel sheet used around a floor is also requested to exhibit rust prevention property together with the reduction of wall thickness of the steel sheet by strengthening the steel sheet. For this end, application of a high strength galvanized steel sheet has been studied. Further, parts around a floor receive strong vibrations during traveling in the same manner as chassis parts (such as lower arms) and hence, high durability against vibrations is requested so that a steel sheet for manufacturing these parts is requested to exhibit excellent fatigue strength.
To ensure plating and chemical conversion treatment property after assembling a vehicle body, it is often the case where punching work is applied to a part around a floor, and it is known that a fatigue property of a portion which is subjected to punching work and constitutes a stress concentration portion deteriorates more compared to fatigue property of a smooth member. Accordingly, there has been a demand for the improvement of fatigue property in punching work.
To satisfy such requirements patent document 1, for example, proposes a high strength cold-rolled steel sheet exhibiting excellent fatigue property and a method of manufacturing the steel sheet, wherein the composition is adjusted to a proper range, the microstructure of the steel sheet is of the multiphase structure of two phases where a main phase is of a ferrite phase and a second phase is of a martensite phase or of a martensite and a retained austenite phases, and a size of Cu grains in the ferrite phase is defined.
Patent document 2 proposes a working-use high strength cold-rolled steel sheet exhibiting excellent fatigue property and a method of manufacturing the steel sheet, wherein the composition is adjusted to proper ranges, the microstructure is of the multiphase microstructure consisting of a ferrite phase, a bainite phase, a retained austenite phase and a martensite phase, and a size of Cu grains in the ferrite phase is optimized.
However, in techniques disclosed in patent document 1 and patent document 2, fatigue strength is evaluated using plane bending fatigue specimens having a notch, wherein the specimen is a machined end surface, and the influence which initial cracks on the end surface brought about by punching work exerts on a fatigue strength is not taken into consideration at all.
Patent document 3 proposes a high strength hot-rolled steel sheet which exhibits excellent fatigue property and excellent stretch-flangeability on a punched end surface where the composition is adjusted to a proper range, alumina-based oxide and titanium nitride are suppressed, and the microstructure adopts a bainitic ferrite as a main phase, and a method of manufacturing the high strength hot-rolled steel sheet.
However, although patent document 3 evaluates fatigue property by taking the punched hole end surface into consideration, patent document 3 completely fails to disclose fatigue property by taking a punched hole end surface of a high-strength galvanized steel sheet into consideration.